| Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
|---|---|---|
| Prev | Chapter 23. Functions | Next |
What makes a variable "local"?
A variable declared as local is one that is visible only within the block of code in which it appears. It has local "scope". In a function, a local variable has meaning only within that function block.
Example 23-12. Local variable visibility
1 #!/bin/bash
2 # Global and local variables inside a function.
3
4 func ()
5 {
6 local loc_var=23 # Declared as local variable.
7 echo # Uses the 'local' builtin.
8 echo "\"loc_var\" in function = $loc_var"
9 global_var=999 # Not declared as local.
10 # Defaults to global.
11 echo "\"global_var\" in function = $global_var"
12 }
13
14 func
15
16 # Now, to see if local variable "loc_var" exists outside function.
17
18 echo
19 echo "\"loc_var\" outside function = $loc_var"
20 # $loc_var outside function =
21 # No, $loc_var not visible globally.
22 echo "\"global_var\" outside function = $global_var"
23 # $global_var outside function = 999
24 # $global_var is visible globally.
25 echo
26
27 exit 0
28 # In contrast to C, a Bash variable declared inside a function
29 #+ is local *only* if declared as such. |
![]() | Before a function is called, all variables declared within the function are invisible outside the body of the function, not just those explicitly declared as local.
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Local variables permit recursion, [1] but this practice generally involves much computational overhead and is definitely not recommended in a shell script. [2]
Example 23-13. Recursion, using a local variable
1 #!/bin/bash
2
3 # factorial
4 # ---------
5
6
7 # Does bash permit recursion?
8 # Well, yes, but...
9 # It's so slow that you gotta have rocks in your head to try it.
10
11
12 MAX_ARG=5
13 E_WRONG_ARGS=65
14 E_RANGE_ERR=66
15
16
17 if [ -z "$1" ]
18 then
19 echo "Usage: `basename $0` number"
20 exit $E_WRONG_ARGS
21 fi
22
23 if [ "$1" -gt $MAX_ARG ]
24 then
25 echo "Out of range (5 is maximum)."
26 # Let's get real now.
27 # If you want greater range than this,
28 #+ rewrite it in a Real Programming Language.
29 exit $E_RANGE_ERR
30 fi
31
32 fact ()
33 {
34 local number=$1
35 # Variable "number" must be declared as local,
36 #+ otherwise this doesn't work.
37 if [ "$number" -eq 0 ]
38 then
39 factorial=1 # Factorial of 0 = 1.
40 else
41 let "decrnum = number - 1"
42 fact $decrnum # Recursive function call (the function calls itself).
43 let "factorial = $number * $?"
44 fi
45
46 return $factorial
47 }
48
49 fact $1
50 echo "Factorial of $1 is $?."
51
52 exit 0 |
See also Example A-16 for an example of recursion in a script. Be aware that recursion is resource-intensive and executes slowly, and is therefore generally not appropriate to use in a script.
| [1] | Herbert Mayer defines recursion as ". . . expressing an algorithm by using a simpler version of that same algorithm . . ." A recursive function is one that calls itself. | |
| [2] | Too many levels of recursion may crash a script with a segfault.
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